The present invention relates to a master timing generator and, more particularly, to a master timing generator employing phase lock loop techniques and capable of tracking an externally generated clock signal and of maintaining desired operation during periods of perturbation of the clock signal by noise or other high-energy disturbances.
In present day communications and switching systems, especially those requiring a large number of precisely-timed operations, it is important that the timing of these operations be properly and correctly executed even in the presence of noise or other similar high-energy disturbances. By way of example, it is common in many digital communications and switching systems to provide a master timing generator from which all timing signals for the system are derived and which is phase locked to an external clock signal to insure that all timing signals derived from the master timing generator are in proper, precise time and phase relationship. The external clock signal which is employed by the master timing generator for phase locking purposes may be conveniently derived in one common approach by extraction from digital data such as digital diphase data having binary information encoded therein by means of standard conditioned diphase modulation techniques. If the external clock signal is corrupted by noise or other high-energy disturbances, the master timing generator will attempt to track the erroneous signal, thereby causing timing perturbations within the system and, consequently, causing serious synchronization problems within the system and degradation of the performance of the system. It is important in the above situation therefore that the presence of noise be quickly and readily detected so that the operation of the system can be altered during the presence of the noise or other appropriate measures taken to compensate for the presence of the noise. Circuitry by which detection of noise may be accomplished and an appropriate alarm signal produced in response to detection of such noise is described in detail in the aforementioned application of Charles R. Marchetti. The present invention is directed to a master timing generator which may be used with an alarm signal and an externally-generated clock signal as described above for maintaining system synchronization even during periods of perturbation of the clock signal by noise or other high-energy disturbances.